1. Field of the Invention
The present invention relates to a method of fabricating an integrated circuit structure. More particularly, the present invention relates to a method of fabricating an interconnect structure.
2. Description of the Related Art
With the rapid reduction in dimension of IC device, the line pitch of interconnect is decreased unceasingly. Because the RC delay effect gets more significant at reduced line pitch, SiO2 with a dielectric constant near 4.0 cannot be used as an IMD material in the interconnect structure. Therefore, low-k materials are widely used instead of SiO2 to reduce the parasitic capacitance, so as to inhibit the RC delay effect and the cross-talking effect and improve the operation speed.
One of current low-k materials is porous low-k material, which can be treated with UV-curing to improve the physical properties thereof, including modulus, hardness and adhesivity, etc. Since an interconnect structure usually includes multiple dielectric layers, multiple cycles of UV curing must be conducted. However, because the porous low-k layers are transparent to UV light, the UV light can pierce the portions of the porous low-k layers not screened by the interconnect structures. Therefore, a lower porous low-k layer is cured more times and receives more UV-irradiation than an upper one, so that the physical properties of the porous low-k layers are not uniform.
FIGS. 1A-1C show respective variations of increased modulus (ΔM), increased hardness (ΔH), increased dielectric constant (ΔK) and increased stress (ΔS) of a porous low-k material with the UV-curing condition. As shown in the figures, the modulus, hardness, dielectric constant and stress of a porous low-k layer all increase with the number of UV-curing cycles. Accordingly, the physical properties of the porous low-k layers are not uniform in the prior art because the numbers of the UV-curing cycles experienced by them are different.